The Mechanics of Open Source

Here is how free software works:
https://pixabay.com/en/forum/official-pixabay-news-2/the-pixabay-license-7823/

people shared various images under creative commons license, and one day site owners suddenly claimed ownership over user images, promising to sue anyone using them under free license.

Okay, I read the thread. They are not saying that they will sue you, this is still shitty though.
My question is, were all the images before 2019 under CC0? Why did they make their own license instead of allowing the use of CC-NC(-SA)?